I would like to thank Old Yeller because after looking at his icon and doing a bit of google research I now finally know what breed my dog was. I mean, he was a mutt, but I never knew what went into him. Now I know! He was in large part Leonberger. Those dogs look exactly like him, including the size, and have the same exact disposition, with the exception that Duke's fur was mostly cream and black - I'm guessing he was also part lab or something.
That's pretty cool to know. Now I know if I ever want another dog (and ever have the space to take care of one) what I should get.
